A survey of dinosaur diversity by clade, age, country and year of description. Michael P. Taylor

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Presentation on theme: "A survey of dinosaur diversity by clade, age, country and year of description. Michael P. Taylor"— Presentation transcript:

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A survey of dinosaur diversity by clade, age, country and year of description. Michael P. Taylor dino@miketaylor.org.uk

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Introduction Understanding dinosaur diversity is essential for understanding Mesozoic ecosystems. There has been relatively little work in this area. The main contributions have all been from Dodson and his collaborators (with another to come at SVP) They have not analysed the record in great detail. The present study analyses diversity data (genus names, ages, dates, countries of origin and relationships) in four different ways.

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Introduction Understanding dinosaur diversity is essential for understanding Mesozoic ecosystems. There has been relatively little work in this area. The main contributions have all been from Dodson and his collaborators (with another to come at SVP) They have not analysed the record in great detail. The present study analyses diversity data (genus names, ages, dates, countries of origin and relationships) in four different ways. => Stand by for lots of numbers!

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Observed and actual diversity The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Observed and actual diversity – Which animals existed? (actual diversity) The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Observed and actual diversity – Which animals existed? (actual diversity) – Which of them were fossilised? The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Observed and actual diversity – Which animals existed? (actual diversity) – Which of them were fossilised? – Which fossils survived until the present? The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Observed and actual diversity – Which animals existed? (actual diversity) – Which of them were fossilised? – Which fossils survived until the present? – Which surviving fossils are in exposed outcrops? The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Observed and actual diversity – Which animals existed? (actual diversity) – Which of them were fossilised? – Which fossils survived until the present? – Which surviving fossils are in exposed outcrops? – Which exposed fossils have been found? The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Observed and actual diversity – Which animals existed? (actual diversity) – Which of them were fossilised? – Which fossils survived until the present? – Which surviving fossils are in exposed outcrops? – Which exposed fossils have been found? – Which found fossils have been collected? The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Observed and actual diversity – Which animals existed? (actual diversity) – Which of them were fossilised? – Which fossils survived until the present? – Which surviving fossils are in exposed outcrops? – Which exposed fossils have been found? – Which found fossils have been collected? – Which collected fossils have been prepared? The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Observed and actual diversity – Which animals existed? (actual diversity) – Which of them were fossilised? – Which fossils survived until the present? – Which surviving fossils are in exposed outcrops? – Which exposed fossils have been found? – Which found fossils have been collected? – Which collected fossils have been prepared? – Which prepared fossils have been studied? The diversity figures we have (observed diversity) are the result of a sequence of chances:

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Materials and methods Database contains dinosaur genera generally considered valid as at the end of 2001. Aves sensu Chiappe is omitted from the database: Clade (Archaeopteryx + modern birds) Analysis program is Free (GNU GPL), and will be made available once the findings have been published. The program DOES NOT run a cladistic analysis: it uses a specified phylogeny, an uncontroversial consensus.

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Controversy over dinosaur genera Dinosaur genera are subject to argument! – Saurophaganax is considered by some to be merely a big Allosaurus. – Others think Allosaurus should be split into multiple genera. Every genus is ultimately a judgement call. The database can only ever be a best approximation to reality. My policy: DON'T GET INVOLVED. I accept the consensus view uncritically.

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The four analyses 1. Phylogenetic. Genus counts aggregated up the tree to high-level nodes. 2. Timeline. Genera counted by the earliest geological age in which they occurred, and aggregated up to epoch and period. 3. Geographical. Genera counted by country of discovery, and aggregated up to continent. 4. Historical. Genera counted by year of naming, and aggregated up to decade.

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Observations on clade diversity Saurischian genera outnumber ornithischians by five to three (282 to 169) Theropods alone outnumber ornithischians! This is surprising given that theropods all look the same (teeth at one end, a tail at the other and a pair of legs sticking down in the middle.) Ornithischians are much more varied in body plan (consider Triceratops, Parasaurolophus and Stegosaurus).

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Observations on clade diversity Saurischian genera outnumber ornithischians by five to three (282 to 169) Theropods alone outnumber ornithischians! This is surprising given that theropods all look the same (teeth at one end, a tail at the other and a pair of legs sticking down in the middle.) Ornithischians are much more varied in body plan (consider Triceratops, Parasaurolophus and Stegosaurus). => The ornithischian renaissance is overdue!

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Carnivores and Herbivores All sauropodomorphs and ornithischians were herbivorous (perhaps excepting a few very basal forms.) Among theropods, ornithomimosaurs and therizinosaurs were probably herbivorous or omnivorous. This leaves 151 carnivorous genera (non- ornithimimosaur, non-therizinosaur theropods) This is one third of the total 451 genera, which seems a high proportion.

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Early dinosaur diversification Dinosaurs appear to have diversified swiftly in the Carnian, the first age in which they appeared. 24 Carnian genera in total: – 6 ornithischians (all basal) – 4 sauropodomorphs (all prosauropods) – 14 theropods 8 basal 6 neotheropods, none of them tetanuran. 12 more new genera in the Norian, including the earliest sauropod, Isanosaurus.

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Diversity trends through time 38 Triassic genera in 21.7 million years from Carnian. => genus density (GD) of 1.75 genera per million years. 124 Jurassic genera in 61.5 million years. => GD = 2.0 289 Cretaceous genera in 79.2 million years. => GD = 3.65 General trend in observed diversity is towards increasing diversity through time. Bias is partly because older fossils have more time in which to be destroyed by processes such as erosion.

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Peaks in dinosaur diversity Three ages are much more diverse than the others: – Kimmeridgian: GD = 11.18 – Maastrichtian: GD = 7.83 – Campanian: GD = 6.80 No other age has a GD greater than 4.0 (Barremian) High diversity in late Cretaceous seems to contradict Dodson 1994's assertion than diversity was declining prior to K/T. This seeming contradiction is probably due to coarser time resolution in the current study.

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Early work was in Europe: first eight genera (28 years) all European until Massospondylus (Lesotho, Africa) became the first non-European dinosaur in 1854. Europe dominated dinosaur genus counts for 65 years from 1825-1889. By 1890, North America had overtaken Europe, and has remained ahead ever since. 45-year gap between Asia's first and second dinosaurs (Titanosaurus in 1877; then three in 1923) In 1993, Asia overtook North America as most diverse continent. Geographical distribution in history

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The overall trend is very obviously towards the more rapid naming of new dinosaur genera. There are large fluctuations between consecutive years. The last year with no new dinosaurs named was 1961; the previous was 1949. So we have had new dinosaurs every year but one of the last half-century. It took 158 years to name the first half of the genera; and 19 years to name the rest – eight times as fast! Observations on dinosaur naming rate

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Apart from a gap in the 30s-60s, the rate of naming appears exponential. These four decades represent the dinosaur dark ages in which palaeontology was largely mammal-oriented. The dark ages ended in the 70s with the Dinosaur renaissance (Ostrom 1969, Bakker 1975) The 56 genera named in the 1970s outnumber all those from the preceding four decades Dinosaur naming rate by decade

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Discussion Why we count genera rather than species. Five reasons for diversity variations between ages and between clades.

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Genus and species Why does this study count genera rather then species? For extant organisms, species may be objectively real and genera merely a convenient abstraction. For extinct organisms, the opposite is more nearly true. Biological concept of species is useless. No-one agrees about the assignment of dinosaur specimens to species, but there is some consensus concerning genera.

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Dinosaur species across the data-set Total number of species is 562 in 451 genera, for an average of 1.25 species per genera. 381 genera (85%) are monospecific. 46 genera have two species, 17 genera have three. Only seven genera have more than three species: – Camarasaurus, Cetiosaurus, Chasmosaurus, Edmontonia (4 species) – Iguanodon, Mamenchisaurus (7 species) – Psittacosaurus (8 species)... And some of these are now squashed (Cetiosaurus).

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Raup (1972) observed a strong correlation between apparent diversity levels of marine invertebrates throughout the Phanerozoic era and the volume of available sediment. (This observation does not make a nice, neat bullet point) Availability of sediment may be the single most significant factor affecting apparent diversity. 1. Geological preservational bias

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Theropods typically have light, hollow bones Sauropodomorphs and ornithischians usually have heavy, solid bones (except sauropod vertebrae) => Theropods should be preserved less often than other dinosaurs But we observe more theropod genera than sauropodomorphs or ornithischians => There must be other factors that outweigh this one. 2. Anatomical preservational bias

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Glamorous clades tend to be split more than others: – Everyone wants to name a new giant Morrison sauropod. – Everyone wants to name a new Tyrannosaur. – No-one wants to name a new basal ornithopod. Examples of over-split big sauropods: – Ultrasauros (Jensen 1985) is a Supersaurus vertebra and a Brachiosaurus scapula (Curtice et al. 1996) – Seismosaurus may be Diplodocus (Lucas in prep.) – Subgenus Giraffatitan (Paul 1988) is not different from Brachiosaurus. 3. Differential splitting/lumping

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Many more papers are published on theropods than on sauropods or ornithischians. This year's JVP abstracts include fourteen on tyrannosaurs alone – this may be more than for all ornithischians combined. Ornithopod specimens collected on expeditions remain in their jackets while the theropods are prepared, studied, described, publicised and recruited to star in Jurassic Park XIV: Wrath of the Raptors. 100 years of Tyrannosaurus symposium coming up next year! (email from Ken Carpenter) 4. Focus of current work

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And now... the SPECIAL MYSTERY GUEST REASON for variation in apparent diversity...

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The diversity of the real ancient ecosystem is the starting point for our observations. But actual diversity is so muddied by preservational and other biases that we need to be VERY CAREFUL in interpreting apparent diversity figures. The results of this study probably tell us more about dinosaur science than about the dinosaurs themselves. 5. Actual diversity

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Theropods seem to be more diverse than either sauropodomorphs or ornithischians. Dinosaur diversity was high in the Carnian, and highest in the Kimmeridgian and late Cretaceous. The USA, China and Mongolia account for more than half of dinosaur genera between them. The rate of naming new dinosaur genera is increasing exponentially. Diversity figures can't be taken at face value because so many biases affect the apparent diversity. Conclusions

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The database analysed in this study is based on that assembled by T. Mike Keesey. Dr. David M. Martill commented extensively on a draft of the manuscript on which this presentation is based. I would never even have started this work without Mathew J. Wedel's encouragement; and it would have been much less useful without his criticism. He should make his mind up. Acknowledgements